Transposition of the great arteries (TGA) is a common congenital cardiovascular malformation characterized by ventriculoarterial discordance. This discordant anatomy is defined by the fact that the aorta arises from the morphologic right ventricle (RV) and the pulmonary artery (PA) from the morphologic left ventricle (LV). The anatomy of TGA is best understood in the context of Van Praagh’s segmental anatomy, a three-letter nomenclature system in which the atria, the cardiac looping, and the position of the great vessels are used to describe cardiac malformations (see Chapter 59). The first letter (S, I, or A) describes atrial situs (solitus, inversus, or ambiguus). The second letter (D or L) refers to the dextro or levo looping of the primitive cardiac tube during fetal development, which determines in turn whether atria and ventricles are concordant [D-looping, right atrium (RA) attaches to RV and left atrium (LA) attaches to LV] or discordant. The third letter describes the relative position of the great vessels. The letter S signifies normally related great vessels, and I denotes normally related but inverted great vessels. D or L describes the rightward or leftward position of the aorta relative to the PA in the setting of transposition. Hence in a normal heart, there is atrial situs solitus, dextro looping, and normally related great vessels, or (S, D, S).

In general, TGA is subclassified in dextro (or D-TGA) and levo (or L-TGA) transposition, referring to the position of the aorta relative to the PA (the last letter of the segmental anatomy classification scheme described above). In TGA, there is generally situs solitus of the atria. D-TGA is invariably associated with D-looping, resulting in a segmental anatomy of (S, D, D). L-TGA is characterized by atrioventricular and ventriculoarterial discordance (RA attaches to LV and LA to RV), and is alternatively termed congenitally corrected TGA. This association of L-looping with l-TGA results in an anatomy coded as (S, L, L). Other extremely rare forms of TGA, including (S, D, L), have been described but are beyond the scope of this chapter. However, the treatment options are generally similar. From a strictly morphologic standpoint, there should only be two forms of transposition: D-TGA, properly termed transposition of the great arteries, and l-TGA, which should be referred to as congenitally corrected transposition.

These are distinct clinical entities, each with several subtypes requiring a number of diverse reconstructive approaches; they are dealt with in separate sections of this textbook. With all forms of D-TGA, there is the need to maintain adequate mixing of oxygenated and deoxygenated blood between the right and left sides of the heart. The circulation in D-TGA is one of two separate circuits, pulmonary and systemic, in parallel (Fig. 74-1). Without communication between the two sides of the heart, deoxygenated blood does not reach the lungs and oxygenated blood does not reach the systemic circulation.

Although TGA was first described by Baillie in 1797,¹ the term transposition is credited to Farre, who used “transposition of the aorta and PA” to describe the malformation in 1814.² The term was further refined by Van Praagh³ in 1971 to define transposition as ventriculoarterial discordance; the term malposition was reserved to positional anomalies of the great arteries observed in other lesions.

Initial surgical therapies for TGA were palliative, aimed at increasing intracardiac mixing and improving systemic oxygen saturation. In 1950, Blalock and Hanlon described a technique for performing an atrial septectomy without cardiopulmonary bypass (CPB) to improve mixing at the atrial level.⁴ Edwards, Bargeron, and Lyons later reported a modification of the Blalock-Hanlon atrial septectomy in which the atrial septum was repositioned posterior to the right pulmonary veins, baffling them to the RA.⁵ Other partial atrial-level switch procedures were devised by Lillihei and Varco,⁶ who anastomosed the right pulmonary veins to the RA and the inferior vena cava (IVC) to the LA, and by Baffes, who performed a similar procedure with an allograft conduit from the IVC to LA.⁷

The first attempts at anatomic correction were directed at switching the two great vessels. Throughout the 1950s, a number of techniques for arterial-level switch operations were described, some including the transfer of one or both coronaries, but most leaving them in the pulmonary circuit. Although they were universally unsuccessful, these pioneering procedures laid the groundwork for the understanding of critical concepts for future procedures, including coronary anatomy and the importance of an LV prepared to handle system workloads. The first successful repairs were once again directed at the atrial level. In 1959, Senning accomplished the first successful atrial-level switch with a series of complex intraatrial baffles of native atrial tissue.⁸ This technique was simplified in 1963 by Mustard with the use of prosthetic material.⁹ The “Mustard procedure” became quickly the treatment of choice for D-TGA and remained such until Jatene reported the first successful arterial switch operation (ASO) in 1975.¹⁰ Jatene and other authors demonstrated the long-term benefits of a true anatomic repair, which could be achieved with acceptable operative mortality. Although the Mustard and Senning operations are no longer used as primary therapy for D-TGA, they remain important for the treatment of L-TGA, which is discussed in greater detail in Chapter 75.

Several theories have been put forth regarding the embryology of D-TGA. Some authors support the theory that the normal spiral septation of the conotruncus does not occur, and that development of a straight septum results in transposition.^11–13 Van Praagh and Van Praagh have postulated that the etiology is persistence of the subaortic conus and reabsorption of the subpulmonary conus, leading to pulmonary-mitral continuity and, ultimately, TGA.¹⁴ Other theories also propose a similar abnormal development of pulmonary-mitral continuity or of the area below the semilunar valves, or an abnormality of hemodynamics and blood flow as the etiology.¹⁵

D-TGA can be subdivided into D-TGA with intact ventricular septum (IVS) (55–60 percent), and D-TGA with ventricular septal defect (VSD) (40–45 percent). In D-TGA/VSD, one-third of the ventricular defects are hemodynamically insignificant. The VSDs are most commonly of the perimembranous or outlet morphologic types. Pulmonic stenosis (PS), causing significant left ventricular outflow tract obstruction (LVOTO), occurs rarely with an IVS. It is not uncommon to measure a preoperative gradient across the left ventricular outflow tract (LVOT) due to shifting of the septum from the high-pressure RV outflow tract (RVOT) to the lower-pressure LVOT in D-TGA/IVS. However, the gradient will resolve with anatomic correction and shift of the septum rightward. Hemodynamically significant PS occurs in approximately 10 percent of D-TGA/VSD and may affect operative decision making.¹⁶

The relative positions of the aorta and PA are variable in TGA (Fig. 74-2). Fortunately, the sinuses of Valsalva of the aorta and PA tend to be arranged in a mirror-image alignment. This arrangement facilitates coronary transfer during the ASO. Similarly, the coronary artery arrangement can be variable. In the current era, most of these coronary artery patterns do not represent an obstacle to surgical correction (as discussed below). To standardize the definition of the coronary anatomy, the sinuses are conventionally numbered 1 and 2. If one imagines oneself standing in the nonfacing aortic sinus looking at the pulmonary root (Fig. 74-3), the right-hand sinus is numbered as 1 and the left-hand sinus as 2. Using this convention (also known as the “Leiden” convention), the coronary anatomy is then described based on the sinuses of origin of the various coronary artery branches. For example, in the most common arrangement, the left anterior descending and circumflex coronary arteries arise from sinus 1 and the right coronary artery (RCA) from sinus 2. This pattern, as seen in Figure 74-3, is described as 1LCx2R. The common coronary arrangements are summarized in Figures 74-4 and 74-5.

As mentioned above, the circulation in D-TGA consists of two circuits in parallel (Fig. 74-1A). Deoxygenated blood is ejected to the body from the RV into the aorta, returning via the venae cavae to the RA and back to the RV. The oxygenated blood cycles from the LV to the PA and through the lungs to return once again to the LA and LV. Thus, intercirculatory shunting is required for mixing to occur. As reflected in Figure 74-1B, the effective pulmonary blood flow (the amount of deoxygenated blood reaching the lungs) is composed of only the anatomic right-to-left shunted blood. Similarly, the effective systemic blood flow (the amount of oxygenated blood reaching the body) is equal to the amount of anatomic left-to-right shunting. There are several potential levels of mixing, including an atrial septal defect (ASD), a VSD, and a patent ductus arteriosus (PDA). In D-TGA/IVS, this is achieved by maintaining a PDA with prostaglandin E₁ (PGE₁) infusion and ensuring an effective atrial-level communication. This may require performance of a blade or balloon atrial septostomy, a fluoroscopically or echocardiographically-guided method of opening the atrial septum, as described by Rashkind in 1966.¹⁷ Rarely, an open atrial septectomy or temporary mechanical support will be required to stabilize a patient prior to an ASO. In D-TGA/IVS and D-TGA/VSD with mild or no PS, there may be adequate mixing at the atrial and/or ventricular levels to allow for the discontinuation of PGE₁. In the presence of significant PS, PGE₁ will be necessary to provide sufficient pulmonary blood flow.

For patients escaping early diagnosis, the evolution of the intermediate-term pathophysiology depends on the anatomy. In the majority of patients with D-TGA/IVS, the closure of the PDA with the resultant loss of mixing will lead to severe cyanosis and cardiovascular decompensation. Although they are generally significantly cyanotic, a minority of patients may survive undiagnosed with D-TGA/IVS owing to adequate atrial-level mixing. In these patients, the LV will become deconditioned as it becomes acclimated to ejecting against the low-resistance pulmonary circulation. After 4 to 6 weeks, this deconditioning progresses to the point where an ASO would result in LV failure, as the LV would be abruptly required to assume systemic workloads. These ventricles require reconditioning prior to an ASO, as detailed below.

Patients with D-TGA and a nonrestrictive VSD will have pulmonary overcirculation, as in the case of any patient with a large systemic-to-pulmonary shunt. However, unlike the typical patient with a large VSD, patients with transposition are at significant risk for the accelerated development of pulmonary vascular obstructive disease (PVOD). As early as 2 months of age, 20 percent of patients with D-TGA and a nonrestrictive VSD will have Heath-Edwards grade 3 or greater histologic pulmonary vascular changes.¹⁸ By 12 months, up to 89 percent will have grade 4 changes.¹⁹ Although the LV remains conditioned due to the pressure load from the nonrestrictive VSD, the development of PVOD may preclude anatomic repair. Patients with D-TGA/VSD/PS can have a more benign natural history, as the appropriate degree of PS leaves them well balanced between adequate effective pulmonary blood flow and protection from PVOD. The LV in patients with D-TGA/VSD/PS remains conditioned due to the pressure load of both the VSD and the PS.

D-TGA is the most common cause of cyanosis in infants and accounts for approximately 10 percent of all congenital cardiovascular malformations.²⁰ The clinical manifestations of a patient with D-TGA are based upon the amount of intercirculatory shunting of the specific anatomy. In the patient with D-TGA/IVS (or a virtually intact septum), cyanosis is essentially universal. Cyanosis is recognized by nursing or physician staff in 56 percent of neonates within the first hour of life and in 92 percent by 1 day.²¹ The remainder of the physical exam is often nondiagnostic. Systolic cardiac murmurs are present in approximately half of the patients, but are usually of grade 2 or less. In the presence of a large VSD, the cyanosis may be very mild and initially overlooked. Signs of congestive heart failure (CHF), including tachycardia and tachypnea, generally become evident by 2 to 6 weeks as pulmonary vascular resistance falls and pulmonary blood flow increases. Murmurs may increase, and other ausculatory findings typically associated with CHF may become evident. These findings may include a grade III to VI pansystolic murmur, a gallop, a middiastolic rumble, and a narrowly split second heart sound with a prominent pulmonary component. The addition of significant PS leads to diminished pulmonary blood flow and severe cyanosis.